Are there any reason(s) that would prevent half of an air-cooled VW engine to be tipped up at a 60-75 degree angle and used as a parallel twin? This assumes Rabbit valve guide seals and some type of custom lower case half/pan with oil pickup.

I believe the VW crankshaft could probably be balanced well enough to identify any showstoppers and prove out that the engine will run with the cylinders pointed skyward. I can still see possible advantages to ultimately using a dedicated (cu$tom) crank though, as that's actually where my initial thoughts were. At that point changing phase angles is easy. Once you start playing with phase angles on the crank, you need to address the cam phasing as well. I'm not sure if the standard VW cam blanks begin with a completely circular profile at the lobes, but it sure would make it easier such that any VW cam grinder could alter the phasing of their standard cam profiles on their standard blanks for a nominal cost...But I don't know if that's the case.

It would also be easier to simply buy a mass produced sports car, rather than attempting to design/build my own from a combination of custom fabricated bits and various off-the-shelf parts...But that would defeat the purpose of the endeavor.

IIRC, ACVW cams don't have phasing in the way you are envisioning because one of the ways they economized the design is that opposite cylinders share the same two cam lobes, which "works" because the engines have a boxer style crankshaft and not a 180 degree V style crankshaft.

Wait, nevermind, I was thinking of turning an ACVW into a 60-70 degree twin. Which won't work unless you have a fanatical devotion to the idea at the expense of all rational arguments. (Which happens)

Well, the Power Of Three says that everything is better in threes, and we know that Subaru perfected the VW engine, and they already kinda-sorta made an inline version of that with the Justy engine, so... completely different solution but pre-engineered?

But, back on topic, the half vw parallel twin has been done 20+(good lord...30+?) years ago. The guy first tried using block off plates for the jugs but ended up with a silly oil capacity like 2 cups. He ended making a custom oil pan. The thing ended up heavier than a stock vw engine. The kicker was it ran funny, and with half the power.

The heaviest car such an engine would be likely to spend some R&D time in, is my current project which is targeted for 1500 lb. A similarly constructed car with the body I want can break 90mph with a 32 hp Kubota turbo diesel, so it could easily still be one of the fastest cars on the road in Seattle. But ultimately it wouldn't have to be limited to such heavy 4-wheeled options either.